1. Field of the Invention
The present invention relates to methods of and apparatus for measuring resistance and more particularly to methods of and apparatus for measuring the resistance between nominally insulated elements at radio frequencies.
2. Technical Considerations and Prior Art
In order to prevent galvanic corrosion of metallic elements, for example, pipes in heterogeneous pipelines for carrying water, oil, gas or the like, it is common practice to impress a small negative DC voltage upon a pipe to supply sufficient electrons thereto so that the pipe becomes a cathode and corrosion reactions are prevented.
It is also common practice to insulate pipe sections from one another by the use of insulating plates or the like. When these insulators deteriorate, break or become contaminated, short circuits occur between adjacent pipe sections through the insulator which divert electrons from one to the other and may render the pipes susceptible to galvanic corrosion. These short circuits may also occur through the insulating plates when adjacent pipe sections are improperly bolted together.
If a DC resistance reading is taken across the adjacent pipe sections through the insulating plate utilizing a conventional DC ohmmeter, the reading on the ohmmeter is representative of the parallel resistances of the adjacent pipe sections both across the insulating plate therebetween and through earth ground, since the pipes are almost always in electrical communication with earth ground. Such readings are virtually meaningless, since the ground to ground DC resistance will cause the meter to read zero or close to zero in every instance. Therefore the insulator must be removed and examined by dismantling the pipe sections.
One prior art solution to the problems associated with detecting short circuits between insulated pipe sections is to apply a radio frequency (RF) voltage between the insulated pipe sections. At radio frequencies the earth will not conduct a current but a contaminated or defective insulator will. Since both the RF and DC resistances of a properly functioning insulator are high and those of a defective insulator are low, this method may be utilized to detect a defective insulator without the necessity of removing it from between the adjacent pipe sections in order to perform a physical examination.
The foregoing method is taught in O. D. Simpson, U.S. Pat. No. 3,163,817. Simpson discloses an RF oscillator, the output of which is coupled to a tank circuit tuned to the oscillator frequency, the output of which is coupled to a milliameter through a detector circuit. A pair of probes or electrodes connected between the output of the oscillator and the input of the tank circuit are placed in electrical contact with the pipe sections on opposite sides of the insulator to determine the RF resistance between the pipe sections.
One substantial drawback of the circuit disclosed in Simpson is that it is essentially a so-called "go" or "no-go" indicator. That is, RF resistances exceeding approximately five ohms produce a full scale reading on the instrument and resistances below five ohms produce non-linear readings on the instrument. The output of the instrument is properly characterized as semi-logrithmic and not linear. The circuit, while capable of determining a totally defective insulator is incapable of determining the soundness of an insulator which has not completely deteriorated but will do so in a relatively short period of time.
Similarly, in many instances where the pipe sections are improperly bolted together, such a condition cannot be determined utilizing the circuit disclosed in Simpson.
There is thus the need for an RF resistance testing circuit which produces a precise linear reading of the RF resistance between adjacent nominally insulated pipe sections. Such a circuit will enable the quality of the insulated coupling to be determined, including a failing insulating plate and an improperly bolted coupling.